1. Field of the Invention
This invention relates to a method for the preparation of high surface area lithium electrodes for use in electrochemical cells. More particularly, it relates to the preparation of porous lithium electrodes by deposition of the lithium from solution in liquid ammonia.
2. Description of the Prior Art
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such s nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an intimate and maximum contact with an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel, graphite or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell. Similarly, U.S. Pat. No. 3,926,669 issued to Auborn on Dec. 16, 1975, discloses that certain liquid inorganic oxyhalides and thiohalides, such as thionyl chloride, sulfuryl chloride and phosphorus oxychloride, can be utilized as cathode depolarizers in such a cell.
Consistent with the disclosure of Maricle et al. in the above-mentioned U.S. Pat. No. 3,567,515, ultra-pure lithium electrodes prepared by vapor deposition of lithium on a glass substrate are stable when placed in direct contact with an electrolyte which comprises sulfur dioxide and in which the dithionite discharge product is soluble. However, we have found that a relatively rapid self-discharge usually occurs when the lithium electrode is fabricated from bulk samples of commercially supplied lithium. For example, when commercial lithium foil is placed in an electrolyte which comprises sulfur dioxide and in which the dithionite discharge product is soluble (dithionite anion is the sulfur dioxide reduction product), one usually observes one or more spots appearing on the lithium surface from which a red to black colored material is released. In some cases, only a few such spots will appear. More typically, however, large areas of the lithium electrode will be covered with such spots. When the lithium electrode is coupled with a carbon cathode, the open circuit voltage of the resulting electrochemical cell decays rapidly as a consequence of the self-discharge process. This self-discharge represents a major obstacle to the construction of a satisfactory electrochemical cell which comprises an active metal anode, a sulfur dioxide cathode depolarizer, and an electrolyte solution in which the dithionite discharge product is soluble. The prior art fails to disclose any method for either the control or prevention of this self-discharge.
High surface area active metal electrodes are highly desirable for use in high energy density cell systems for a variety of reasons. For example, high surface area electrodes provide higher currents and, accordingly, permit the delivery of greater power from both primary (nonrechargeable) and secondary (rechargeable) cells during discharge. In those cases wherein an insoluble product is produced during discharge and deposited on the active metal electrode, high surface area electrodes provide a greater discharge capacity before the active surface becomes coated. Further, high surface area electrodes are generally porous in nature. In rechargeable secondary cells, this porous nature permits the retention of any insoluble discharge product within the pores and thus facilitates the recharge process. This retention of the discharge product near the electrode in secondary cells serves to reduce electrode shape change during recharge, thereby extending cycle life.
The alkali metals and, to a lesser degree, calcium, strontium, barium, europium and ytterbium are soluble in liquid ammonia. The resulting solutions are blue in color when dilute and are bronze or metallic appearing at metal concentrations of about 3 molar or above. In the dilute solutions, it is generally believed that the metal is dissociated into solvated metal ions and electrons. The more concentrated bronze appearing solutions possess physical properties, such as a metallic luster and an exceedingly high electrical conductivity, which resemble those of liquid metals. The foregoing metals are also soluble, to varying degrees, in amines. In addition, it is known that lithium, sodium and potassium are soluble in hexamethylphosphoramide, a compound which has the formula [(CH.sub.3).sub.2 N].sub.3 PO.
A procedure for the preparation of alkali metal and alkaline earth metal dispersions in hydrocarbon liquids has been developed which is based on the solubility of these metals in liquid ammonia. This procedure involves dispersing an ammonia solution of the metal in a hydrocarbon liquid and allowing the ammonia to evaporate (see I. Fatt and M. Tashima, Alkali Metal Dispersions, D. Van Nostrand Co., Inc., 1961, pp. 65-70). Similarly, C. M. Stupak disclosed at the Colloque Weyl (June 26-July 1, 1983, at Pacific Grove, Calif.) that lithium can be purified by filtering a saturated ammonia solution of this metal and recovering the metal by evaporation of the ammonia.